Many metal parts for use in an automobile and a home appliance are made by deforming flat metal plates into a variety of shapes. In a case of mass-producing such a metal part, a press forming method of deforming the metal plate by using a press machine and a metal die is widely used. In usual, the metal plate before being machined is flat, and accordingly, in a case of deforming the metal plate into a complicated three-dimensional shape, the metal plate expands and contracts to a shape in match with the three-dimensional shape. However, as a part shape (the three-dimensional shape) becomes more complicated, it becomes more difficult to give the expansion and the contraction, which are allowed to correspond to the three-dimensional shape, to the metal plate. In particular, in a case where the metal plate for use is a high-tensile steel plate or an aluminum alloy plate with a tensile strength of 590 MPa or more, and in a case where the metal plate is a difficult-to-form member poor in ductility and Lankford value, it tends to be difficult to form the metal plate into a free three-dimensional shape.
In a case where the metal plate cannot be given the expansion and contraction in match with the three-dimensional shape in an event of the press forming, such a forming defect as a crack and a wrinkle occurs in the metal plate. For example, in an event where the metal plate is deformed to the three-dimensional shape, the metal plate cannot help expanding more than necessary in a region in which a length of the metal plate falls short and an amount of shortage cannot be compensated from a periphery thereof, and as a result, the crack occurs when the metal plate is pulled beyond ductility thereof. Meanwhile, in a case where the length of the metal plate must decrease in the event where the metal plate is deformed to the three-dimensional shape, and in a region into which a material flows excessively from a periphery in that event, the wrinkle is prone to occur in a press-formed part already obtained by the forming.
As an example of a part shape for which it is difficult to perform the press forming, as illustrated in FIG. 1 and FIG. 2, there is a part shape, in which a cross-sectional shape is a U shape or a hat shape, the part shape having a bent portion in a direction intersecting such a cross section perpendicularly (that is, a longitudinal direction of the part; hereinafter, sometimes simply referred to as a longitudinal direction). In a case of forming the flat metal plate into such a complicated part shape, the crack and the wrinkle are prone to occur in the press-formed part, which is already obtained by the forming, due to tensile deformation and compression deformation, which occur in the above-described bent portion.
Here, as a method for suppressing the occurrence of the crack and the wrinkle in the press forming, there is a method of forming the metal plate while applying tension thereto by sandwiching the metal plate by metal dies as in draw forming. In this method, when the tension applied to the metal plate is increased, it becomes difficult for the metal plate to flow into a three-dimensional shape portion excessively, and the occurrence of the wrinkle can be prevented. On the contrary, when the tension applied to the metal plate is decreased, it becomes easy for the metal plate to flow into the three-dimensional shape portion, and the occurrence of the crack can be prevented.
Then, PTL 1 proposes to change the tension, which is required during the forming, appropriately by using an embossed shape called beads. Moreover, PTL 2 proposes to enhance rigidity of end portions in the width direction of the metal plate, to thereby enhance resistance of the metal plate to contraction deformation when the metal plate is flown, and to suppress the occurrence of the wrinkle.